Many industrial processes require precise control of film thickness. In semiconductor processing, for example, a semiconductor wafer is fabricated in which one or more layers of material from the group comprising metals, metal oxides, insulators, silicon dioxide (SiO2), silicon nitride (SiN), polysilicon or the like, are stacked on top of one another over a substrate, made of a material such as silicon. Often, these layers are added through a process known as chemical vapor deposition (CVD), or removed by etching or removed by polishing through a process known as chemical mechanical polishing (CMP). The level of precision required can range from 0.0001 μm (less than an atom thick) to 0.1 μm (hundreds of atoms thick).
To determine the accuracy of these processes after they occur, or to determine the amount of material to be added or removed by each process, it is advantageous to measure the thickness of the layers on each product wafer (i.e., on each wafer produced that contains partially processed or fully processed and saleable product), which is generally patterned with features on the order of 0.1 μm to 10 μm wide. Because the areas covered by these features are generally unsuitable for measurement of film properties, specific measurement sites called “pads” are provided at various locations on the wafer. To minimize the area on the wafer that is taken up by these measurement pads, they are made to be very small, usually about 100 μm by 100 μm square. This small pad size presents a challenge for the film measurement equipment, both in measurement spot size and in locating the measurement pads on the large patterned wafer. A measurement spot size of an optical system refers to the size of a portion of an object being measured that is imaged onto a single pixel of an imaging detector positioned in an image plane of the optical system.
To date, though its desirable effects on product yield and throughput are widely recognized, thickness measurements are only made after certain critical process steps, and then generally only on a small percentage of wafers. This is because current systems that measure thickness on patterned wafers are slow, complex, expensive, and require substantial space in the semiconductor fabrication cleanroom.
Spectral reflectance is the most widely used technique for measuring thin-film thickness on both patterned and unpatterned semiconductor wafers. Conventional systems for measuring thickness on patterned wafers employ high-magnification microscope optics along with pattern recognition software and mechanical translation equipment to find and measure the spectral reflectance at predetermined measurement pad locations. Examples of this type of system are those manufactured by Nanometrics, Inc., and KLA-Tencor. Such systems are too slow to be used concurrently with semiconductor processing, so the rate of semiconductor processing must be slowed down to permit film monitoring. The result is a reduced throughput of semiconductor processing and hence higher cost.
A newer method for measuring thickness of patterned films is described in U.S. Pat. No. 5,436,725. This method uses a CCD camera to image the spectral reflectance of a full patterned wafer by sequentially illuminating the wafer with different wavelengths of monochromatic light. Because the resolution and speed of available CCD imagers are limited, higher magnification sub-images of the wafer are required to resolve the measurement pads. These additional sub-images require more time to acquire and also require complex moving lens systems and mechanical translation equipment. The result is a questionable advantage in speed and performance over traditional microscope/pattern recognition-based spectral reflectance systems.
Ellipsometry is another well-known technique for measuring thin film thickness. This technique involves measuring the reflectance of p-polarized and s-polarized light incident on a sample. Systems exploiting this technique include a light source, a first polarizer to establish the polarization of light, a sample to be tested, a second polarizer (often referred to as an analyzer) that analyzes the polarization of light reflected from the sample, and a detector to record the analyzed light. Companies such as J. A. Woolam, Inc. (Lincoln, Nebr.) and Rudolph Technologies, Inc. (Flanders, N.J.) manufacture ellipsometer systems.
Taking reflectance measurements from measurement pads on semiconductor wafers requires that the measurement pads first be located relative to the measurement apparatus. In traditional microscope-based systems this is usually done by using a video imaging device such as a CCD camera in conjunction with pattern recognition software, which compares the wafer image to a known (learned) image, to determine the wafer orientation. Once the wafer orientation is determined, the microscope objective can then be moved relative to the wafer so that it is positioned above the measurement pad and then the reflectance data taken. With spectral imaging techniques, the problem is similar, except that the imaging device and the reflectance measurement apparatus are the same, and the reflectance spectra are taken from the image in software after the pattern recognition step has determined the wafer orientation.
A problem in employing the methods described above is that images taken of nominally identical patterns on a wafer can vary greatly depending upon the film thicknesses present. The variations may arise either because the images are taken from different areas on the same wafer that have different film thicknesses, or because they are taken from different times in the process sequence, which results in different film stacks and/or film thicknesses. For example, fabrication systems that employ processing steps such as chemical mechanical polishing (CMP) can cause contrast variations to occur in images taken at the same location but at different times. In another example, shown in FIG. 39 herein, two images are taken from the same wafer, but in different locations that have different film thicknesses. In these images, the measurement pads appear as a column of square boxes in the center. A noticeable contrast variation occurs between similar areas on the two images. When the image contrast of the wafer changes such that shapes and edges invert or disappear, the software can no longer reliably determine wafer orientation. Thus, pattern recognition software that relies on edge and/or shape detection can be ineffective or unreliable during wafer processing.